I payed no more than 20$ originally for both of my 85e Ursi. But I also payed ridiculous amounts of money for my first Blazvatan and Aa'une, an amount high enough that I am embarrassed to tell, so I might not be the best example.

Nowadays, 20$ for a card is a bit out of hand unless that stats or card are ridiculously rare. Droskin is the only card, really, that you should be paying more than 30$ for. Max statted meta cards are really going to be the only ones in the 20-30$ range, and the 10-20$ range will be the max priority statted meta cards(for example my Tartarek Psi has max c/w/e). 10$ and under gets a little more tricky to determine, but it isn't as big of a deal either.

REALLY what it comes down to is what your buyer considers them worth, but at least with the above info you have some kind of ground as to not get ripped off.